Abstract By combining turbidimetric titration, dynamic light scattering (DLS), and zeta potential methods, we demonstrated that the adsorption of bovine serum albumin (BSA) in annealed cationic spherical polyelectrolyte brushes (SPB) can be controlled by modulating the pH, ionic strength, BSA concentration of the mixed solution, and SPB thickness. The SPB consist of a polystyrene core with a diameter around 80nm and a dense shell of poly (2-aminoethylmethacrylate hydrochloride) (PAEMH) with a thickness from 10 to 60nm covalently attached on the core surface. Results revealed the existence of three pH regions, corresponding to (1) adsorption of BSA in SPB, (2) aggregation of SPB induced by BSA adsorption, and (3) desorption of BSA from SPB. The extent of the pH regions can be modulated by ionic strength, BSA concentration, or SPB thickness. Adsorption measurements demonstrated that the adsorbed amount of BSA in SPB was affected by pH, ionic strength, and SPB thickness. These findings lay the foundation for protein separation by SPB.